At present, in applications in the field of domestic electrical appliances, OA equipment, and other electrical apparatus, demands for ever higher levels of performance and compactification are being met by cutting and machining rare-earth-type sintered magnets, which have superior magnetic properties, to desired shapes, or forming rare-earth-type bonded magnets to desired shapes, instead of using conventional hard ferrite magnets.
Although high-precision permanent magnets fabricated by cutting and machining sintered magnets give high performance, they have a drawback in that they are very expensive compared to conventional hard ferrite magnets, regardless of the type of material used. Moreover, the minimum processing thickness is limited to approximately 0.2 mm, and it is not possible to manufacture a magnet having a smaller thickness than this.
Bonded magnets, on the other hand, are manufactured, for example, into flat magnets having a diameter of 3 mm and a thickness of 0.3 mm, and are used as permanent magnets for miniature stepping motors in clocks, but since this involves pressure moulding of resin and magnetic particles having a grain size of 50 .mu.m.about.300 .mu.m, it is difficult to obtain a moulded product having a thickness of less than 0.1 mm, for example. In particular, in ring magnets, there is a minimum thickness limit of approximately 0.8 mm, when using a method where the magnet is compressed by a punch in a direction perpendicular to its thickness.
Moreover, in cases where a magnet is formed having a long direction in the direction of compression, the pressure is not transmitted uniformly due to frictional resistance between the magnetic powder and the die casting surface, and it is difficult to mould a long product having a small thickness. Recently, it has been reported that long ring magnets having a thickness of 0.5 mm can be manufactured by extrusion moulding of a bonded magnet, but the magnetic properties are degraded in proportion of the ratio of resin, and even at most, the residual density of magnetic flux Br is 7 kG and the maximum energy product (BH)max is in the order of 9.9 MGOe.
Conventionally, 2-17 type Sm-Co alloy powder is used as the magnetic powder for a bonded magnet, but recently, Nd--Fe--B type alloy powder manufactured by an HDDR method has been used as the magnetic powder for bonded magnets. The aforementioned powders are both magnetic powders which were developed for use in bonded magnets, and it is not possible to form these powders into permanent magnets by processing the powders themselves.
Moreover, at present, in many cases, an isotropic magnetic powder, such as an Nd--Fe--B type powder, manufactured by a molten alloy rapid cooling method, is used as a bonded magnet powder, but since this material is obtained in the formed of thin flakes consisting of a crystalline material by means of rapid cooling of a molten alloy, it is extremely brittle and cannot be formed to any desired shape by flexible bending or a punching process, and hence it is limited to use as a magnetic powder for bonded magnets.
Moreover, although cost reductions can be achieved by manufacturing a bonded magnet since a desired shape can be obtained without the cutting processing required for sintered magnets, because Nd--Fe--B magnetic powder of average particle size approximately 150 .mu.m is combined by means of a resin, in cases where the magnetic powder may disperse readily, for instance, when the magnet is used in an HDD motor, there is a high risk that the recording medium will be damaged by the dispersed powder, and it is necessary to take countermeasures, such as surface coating, or the like, in order to prevent dispersion of the powder.
Moreover, since the Nd--Fe--B magnetic powder is a crushed powder obtained by crushing rapidly cooled alloy thin strip, the cut surfaces of the crushed powder are highly active and oxidize readily compared to the surfaces of rapidly cooled thin strip, and if no surface coating is provided in order to prevent oxidation and the powder is left for 1000 hours under environmental conditions of 80.degree. C. and relative humidity 90%, then in the case of a magnet having a permanence coefficient Pc of 1, not only will the magnetic flux density be reduced by approximately 2% due to the effects of oxidation, but rusting will also occur on the surface, causing powder to become detached.
On the other hand, recently, in an Nd--Fe--B type magnet, a magnetic material having as a main phase an Fe.sub.3 B type compound in an approximate Nd.sub.4 Fe.sub.77 B.sub.19 (at %) composition has been proposed by R. Coehoorn, et. al., J. de Phys. C8, 1988, pp.669.about.670), and the technical details thereof have been disclosed in U.S. Pat. No. 4,935,074. Prior to this, Koon had proposed, in U.S. Pat. No. 4,402,770, a method for manufacturing a permanent magnet composed of very fine crystals by applying crystallization heat treatment to an La--R--B--Fe amorphous alloy containing La as an essential element.
In recent years, it has been reported that thin flakes having hard magnetic properties can be obtained by heat treatment at 700.degree. C. of amorphous flakes obtained by spraying molten Nd--Fe--B--V--Si alloy containing 3.8 at %.about.3.9 at % of Nd onto a rotating Cu roll, as disclosed by Richter et. al. in EP Patent 558691 B1. These permanent magnetic materials have a semi-stable structure comprising a mixed crystal composition combining an Fe.sub.3 B phase, which is a soft magnetic material, and an R.sub.2 Fe.sub.14 B phase, which is a hard magnetic material, obtained by applying crystallization heat treatment to amorphous flakes having a thickness of 20 .mu.m.about.60 .mu.m.
These permanent magnetic materials have a Br figure of approximately 10 kG and iHc of 2 kOe.about.3 kOe, and since their content of Nd, which is an expensive material, is of the order of 4 at %, the cost of the combined starting materials is less expensive that Nd--Fe--B magnets which have a main phase of Nd.sub.2 Fe.sub.14 B, and hence they are superior to conventional rare-earth magnets in terms of cost-to-performance ratio and have been proposed as alternative materials to hard ferrite magnets, although they are restricted to use as bonded magnets, similarly to conventional Nd--Fe--B bonded magnets having a main phase of Nd.sub.2 Fe.sub.14 B.
However, even when a magnetic powder having high magnetic properties is used in a bonded magnet, since it is difficult to raise the content ratio of the magnetic powder above 80%, high magnetic properties cannot be expected of a bonded magnet, and in the case of small-scale bonded magnets, in particular, a maximum isotropic figure of approximately 10 MGOe is obtained.